CollapsiblePanelExtender ImageControlID in asp.net ajax

CollapsiblePanelExtender ImageControlID
CollapsiblePanelExtenderImageControlID.aspx

<%@ Page Language="C#" AutoEventWireup="true" %>

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

<!DOCTYPE html>
<script runat="server">
    protected void Page_Load(object sender, EventArgs e)
    {
        CollapsiblePanelExtender1.ImageControlID = "Image1";
    }
</script>
<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
    <title>Ajax CollapsiblePanelExtender - How to set change ImageControlID programmatically in CollapsiblePanelExtender</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <h2 style="color:SteelBlue; font-style:italic;">
            Ajax CollapsiblePanelExtender - How to set change
            <br /> ImageControlID programmatically in
            <br /> CollapsiblePanelExtender in asp.net
        </h2>
        <hr width="500" align="left" color="DarkTurquoise" />
        <asp:ScriptManager 
            ID="ScriptManager1"
            runat="server"
            >
        </asp:ScriptManager>
        <cc1:CollapsiblePanelExtender 
            ID="CollapsiblePanelExtender1"
            runat="server"
            TargetControlID="Panel2"
            ExpandControlID="Panel1"
            CollapseControlID="Panel1"
            Collapsed="true"
            ExpandedImage="~/Image/Expand.gif"
            CollapsedImage="~/Image/Collapse.gif"
            >
        </cc1:CollapsiblePanelExtender>
        <asp:Image ID="Image1" runat="server" />
        <br />
        <asp:Panel 
            ID="Panel1" 
            runat="server"
            Width="125"
            BackColor="Orchid"
            BorderColor="MediumOrchid"
            BorderWidth="2"
            HorizontalAlign="Center"
            BorderStyle="Solid"
            >
            <asp:Image 
                ID="Image2" 
                runat="server" 
                ImageUrl="~/Image/Hippopotamus.jpg"
                Height="75"
                />
        </asp:Panel>
        <asp:Panel 
            ID="Panel2" 
            runat="server"
            Width="550"
            BorderColor="Purple"
            BorderWidth="2"
            BackColor="MediumPurple"
            HorizontalAlign="Center"
            >
            <asp:Image 
                ID="Image3" 
                runat="server" 
                ImageUrl="~/Image/Hippopotamus.jpg"
                Height="325"
                />
        </asp:Panel>
    </div>
    </form>
</body>
</html>
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